First, immunoassay is described below as an example of sample analysis.
Immunological testing uses specific reactions between an antigen and an antibody, to detect or quantitatively measure the antibody or antigen in a body fluid such as plasma, serum, or urine, and diagnose a disease, a pathological state, and the like. Enzyme-linked immunosorbent assay (ELISA) is available as a typical method of immunological testing. In the ELISA method, an antibody against an antigen which is to be assayed is immobilized as a first antibody in a bottom portion of a container and then a sample of plasma, serum, or urine is added in the container to bind the antigen in the sample to the first antibody. In addition, an antibody with a label bound thereto is further bound as a second antibody to the antigen bounded to the first antibody, and then a signal emitted from the label can be detected. The presence and quantity of antigen in the sample is thus assayed. A fluorescent substance, for example, is used as the label. In this case, intensity of the light emitted from the fluorescent substance increases in proportion to the number of second antibodies having the label bounded thereto, that is, the quantity of antigen, and using a photomultiplier or the like to detect the light emitted from the fluorescent substance will allow quantification of the antigen in the sample.
In a more specific example of an immunological test device using an ELISA method, magnetic particles are used as a solid phase, with a first antibody being immobilized to the surface of the magnetic particles. A substance with a fluorescent dye bounded thereto as a label, that is, a fluorochrome-labeled substance is bound to a second antibody. When an antigen-antibody reaction is caused by mixing a biologically derived detection substance (antigen) and the magnetic particles having the first antibody immobilized thereto, a specific antigen contained in the sample binds to the magnetic particles via the first antibody. When the second antibody is further made to react, the fluorochrome-labeled substance binds to the magnetic particles via the second antibody, the antigen, and the first antibody. The quantity of fluorochrome-labeled substance depends upon, and thus increases or decreases with, the quantity of detection substance contained in the sample, that is, upon the quantity of antigen.
After the magnetic particles with the detection substance bounded thereto have been captured onto a specific region, a laser or the like is activated for that solution. The fluorochrome-labeled substance bound to the magnetic particles will then emit light. Detection of the intensity of the emitted light will allow the quantity of detection substance in the sample, that is, the quantity of antigen, to be assayed. That is to say, the antigen in the sample can be quantitatively measured.
To conduct highly sensitive immunoassay, the magnetic particles with the detection substance (antigen) bounded thereto are captured onto a specific region using a magnet or the like, and while the magnetic particles remain captured, the solution that contains antibodies unbound to the antigen of interest is replaced with a new solution. That is to say, bound antigen-antibody aggregates and free (unbound) ones are separated from each other. This operation is called “bound/free (B/F) separation”.
Methods for causing magnetic particles to be captured onto a capturing region in an analyzing device are described in Patent Documents 1 and 2.